The conventional mobile radio systems are typically intended to support a single radio interface between base stations and mobile stations all over the mobile network. Similarly, a single signalling interface supporting the radio interface is employed between a mobile switch and a base station system, i.e. a base station controller and base stations. In future mobile radio systems, however, it may be necessary to support multiple radio interfaces within a single mobile radio network. To support these multiple radio interfaces, it is necessary to support multiple signalling protocols within a single signalling interface between the base station controller and the mobile switch. This is the case, when base stations supporting different air interfaces are connected via a common base station controller to a mobile switch. If these different radio interfaces utilize different signalling protocols, the mobile switch must be capable of recognition to which signalling protocol the received message belongs. This is required in order to facilitate the mobile switch to select the right signalling protocol for further communication between network elements. Naturally, similar separation is needed also for signalling messages from the mobile switch to the base station system.
One example of conventional mobile radio systems is Paneuropean digital mobile radio system GSM (Global System for Mobile Communication). The interface between base station controller and the mobile switch is called A-interface in the GSM system. Interfaces very much similar to the GSM A-interface will, apparently, be standardized for personal communications systems (PCS). It is expected that supporting multiple radio interfaces will be needed also in PCSs.